In a bowl place chicken, mushrooms, onions and lightly cover with EVOO and sprinkle with desired amount of garlic salt and pepper. Toss together and place on a metal skewer in desired order including pineapple. Lightly drizzle with balsamic vinegar and place on preheated grill at 400 degrees. Cook for 6-10 minutes until cooked through. Turn once during the cooking process.